PMD, PDL, and PDG are significant penalty sources in high-speed (e.g., 10 Gb/s and 40 Gb/s) transmissions. PMD compensation (PMDC) is normally desirable to increase system tolerance to PMD. However, due to the stochastic nature of PMD and its wavelength dependence, PMDC is normally implemented for each wavelength channel individually, and is thus, generally not cost-effective. Various prior art methods have been proposed to achieve PMDC simultaneously for multiple WDM channels. Channel switching is one technique that has been proposed to mitigate the overall PMD penalty in a WDM system. However, such systems sacrifice system capacity due to the use of extra channels for PMD protection. Multi-channel PMDC before wavelength de-multiplexing has also been proposed to compensate for the PMD degradation in the WDM channel having the most severe PMD. However, such a mitigation scheme may cause degradation of other channels.
Another scheme for a multi-channel shared PMDC has been proposed in which the most degraded channel is switched, by optical or electrical means, to a path connected to the shared PMDC. However, the speed of such a PMDC solution is limited by the speed of the optical or electrical switching.
In each of the prior art PMDC schemes discussed above, undesirable PMD induced system outages (during which the PMD penalty exceeds its pre-allocated system margin) are present, though reduced.
Forward-error-correction (FEC) is an effective technique for increasing system margin cost-effectively. It has been determined, however, that FEC cannot extend the tolerable PMD for a fixed PMD penalty at a given average bit-error-rate (BER), even though the additional margin provided by FEC can be used to increase the PMD tolerance. It has been suggested that sufficient interleaving in FEC may increase PMD tolerance. However, there is no known practical method to provide the deep interleaving needed to avoid a PMD outage, which may last minutes or longer in practical systems.
Although distributed polarization scrambling has been proposed as an effective technique for mitigation of PMD/PDL/PDG penalties, a need exists for optimized and cost effective designs for providing polarization scrambling to mitigate PMD/PDL/PDG.